Beneficiation of ores



Nov. 29, 1960 Filed April 16, 1957 ORE a4. 82%

D. WESTON BENEFICIATION 0F ORES V 3 Sheets-Sheet 2 INVEN TOR. 04 W0 WESTON ,47'roz/vsys Nov. 29, 1960 D. WESTON 62,

BENEFICIATION OF ORES Filed April 16, 1957 5 Sheets-Sheet 3 ORE 34. 7%/0 114 IL 5 i006 p/vc.

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INVENTOR. JAM 0 WE 7'0 United States Patent F BENEFICIATION F onns DavidWeston, 129 Adelaid St. W., Toronto 1, Ontario, Canada Filed Apr. 16,1957, Ser. No. 653,240

3 Claims. (Cl. 241-80) This invention relates to methods of millingmaterial in dry milling circuits and more particularly to a method ofdry milling embodying concentration of values and the production of abeneficiated milling circuit product.

The method of the present invention while it applies particularly to thebenefication of ores containing com ponents amenable to magneticseparation has broad application to the beneficiation of all orematerials which are amenable to any form of concentration at a stage ofreduction at which the values are contained in composite particles,rather than primary crystalline particles.

It has hitherto been the practice with such materials to continuecomminution to a point where substantially all of the values have beenreduced at least to the primary crystalline grain size (i.e. theparticle size wherein the minerals are effectively liberated) and thento concentrate the values from the resulting primarily mechanicalmixture of particles of values and gangue. The reduction of the materialmay be carried out according to these known methods either in a singlepass through a series of reduction units or by employing one or morereduction units in which an oversize fraction of the product isrecirculated.

' I have now found that in the case of many minerals and notably in thecase of magnetic iron ores the distribution of values in particlesconsisting of aggregates of primary crystals is nonuniform in nature sothat a substantial percentage of such particles consist for the most ofaggregates of crystals of values and a substantial percentage of suchparticles consist for the most part of aggregates of substantiallybarren gangue particles. I have further found that in specific cases itis possible to take advantage of this fact to produce a higher grade ofconcentrate with a concurrent saving in milling costs by removing asubstantial proportion of the material from the milling circuit at asize range considerably above that desired in the final product,subjecting the thus removed material to concentration, e.g. magneticconcentration in the case of magnetic iron ores, and then returning theconcentrate to the inlet side of the milling circuit along with thenormal feed thereto, and in many cases discarding the tailing.

Preferably, according to the present invention,.the milling operation isconducted in'a combined dry crushing and grinding unit whereinthe'product is extracted in a stream of air. The mill is so adjustedthat a substantial proportion of the material carried away from the millby the airstream is within a size range coarser than that desired as afinal product and a fraction (which may be considered a middlings inrelation to the operation being conducted) is removed by airand/ormechanical classification and passed through a dry magneticconcentrator. The concentrate is returned to the'inlet side of the millwhile the tailing may usually be discarded as a coarse tailingcontaining only a very small amount of values.

' The invention and its operation will be described in some detail asapplied to the beneficiation of magnetic iron ores in the followingdetailed specification in which reference will be had to theaccompanying drawings wherein:

, 2,962,231 Patented Nov. 29, 1960 Figure l is a schematic illustrationof a typical flow sheet for the beneficiation of magnetic iron oresoperating according to previously known principles.

Figure 2 is a schematic illustration of a flow sheet according to oneembodiment of the present invention.

Figure 3 is a schematic illustration of an alternative embodiment of thepresent invention.

Referring now more particularly to the drawings in the conventionalmethod of operation as illustrated in Figure l which is given forpurposes of comparison, feed 10 is delivered to the combined drycrushing and grinding unit 11 which is an airswept mill discharging intoan air classifier 12 which removes the oversize and permits the mainproduct to pass to cyclone 13. After a deposit of the main product incyclone 13 fines are collected in the multicyclone 14 and dust which istoo fine for collection in the multicyclone 14 is collected in the bagfilter 15. The air classifier 12 discharges on to the mechanical screen16 and the oversize from the said screen is returned to the inlet sideof the mill. The through product from screen 16 is passed to magneticseparator 17 which produces a final tailing 18 of low metal content(20.0% Fe) and a rough concentrate of higher metal content which ispassed to a second magnetic concentrator 19, which latter produces afinal coarse concentrate 20 containing 51.3% Fe and a minor proportionof tailings and middlings which are combined and recirculated to theinlet side of the mill 11. The main product from cyclone 13 is passed toa magnetic concentrator 21 which produces a final fine concentrate 22 of63.5% Fe and fine tails 23 containing 17.0% Fe.

Typical results obtained operating in accordance with the flow sheet ofFigure 1 are indicated in the following example. 7

Example I.A 5 by 2' combined dry crushing and grinding unit was broughtinto operating balance in accordance with the flow sheet illustrated inFigure 1 using as feed a low grade magnetic iron ore obtained from a deposit in Northern Quebec with a feed rate of 057 ton per hour and avelocity of air through the mill corresponding to a water gauge reading1.5" at the mill outlet. The screen 16 was a 20 mesh screen and underbalanced conditions the +20 mesh material returned to the mill from thetop of screen 16 amounted to 5% by weight of the feed. The combinedcoarse tails and middlings re: turned from the magnetic concentrator 19amounted to 2% of the total feed. The results'of this run areillustrated in Table I:

Table I v r Ironin Product lSoluble iron. in Percent product Weiht 5Final Product Based on Total 'Percent. Percent Percent Percent Feed Ironin 'Weizht Iron in Wei ht 7 Product oi Feed Product of Feed Coarse Tails18. 25.6 20.0 5. 1 7. 0 1. 8 (approx.) Coarse Gone. 20.... 20.1 51. 3.10. 3 50.0 10.0 Fine Gone. 22. 17. 5 .63. 3 11.1. 62.3 10. Multicvcloue24 4. 4 22. 2 0. 98 14.0 0. 62 Fine Tails 23 26. 5 17.0 4. 5 9.0 2. 38Dust; 25 5. 9 24.4 1. 43 17.0 1.0

The same equipment was then operated under balanced c onditionsinaccordance with the methodof the present invention, and the results ofthese runs using the same ore is set forth in Examples 2 and 3. HExample 2 (see Figure 2).The mill was brought into balance with a feedrate of 0.54 ton'per hour with a, velocityofair through the millcorresponding to 1.9' ',wa-; ter gauge at the outlet side thereof. Inthis case, the amount of +20 mesh returned to the mill from the top ofscreen 16 amounted to 4% of the mill feed. In this case, the magneticconcentrator 17 produces a middlings and a tailings which are combinedand returned to the inlet side of the mill 11 and the concentrate fromconcentrator 17 is cleaned in asecond magnetic concentrator 19 whichproduces a coarse concentrate which is returned to the inlet side of themill anda final coarse tailing 26 which is discarded. The main productfrom the cyclone 13 passes to magnetic concentrator 21 which produces afine concentrate 22 and a fine tailing which is passed to the furthermagnetic concentrator 21a where it is separated into a final finetailing 23 and a middling which is recycled to a point just in advanceof the cyclone 13.

With conditions in balance the amount of coarse concentrate andmiddlings recycled to the inlet of mill 11 from the magneticconcentrators 17 and 19 amounted to 50% of the weight of the feed 10.Results of a typical run are shown in Table II.

It will be noted from Table II with respect to line concentrate 22 that32.5% by weight of the product is obtained with an improved iron contentof 67.5% by weight as compared to only 17.5% by weight of fineconcentrate 22 in Table I with the lower iron content of 63.3% byweight. Thus, almost double the amount of a higher premium concentrateis obtained with the invention. Similar comparisons are indicated as tothe soluble iron content.

Example 3.-The same equipment was brought into balance with the same orein accordance with the method of operation illustrated in Figure 3 whichdiffers from that shown in Figure 2 in that the middlings from magneticconcentrator 21a is returned to the inlet side of the mill rather thanto a point in the collection circuit. In this case under balancedconditions the amount of the material returned from the concentrators17, 19, 21a to the inlet side of the mill amounted to 100% of the feedwhile the mesh returned to the mill from screen 16 amounted to 4% of thefeed 10. Under balanced conditions, the capacity of the mill was 0.54ton per hour with a velocity of air through the mill corresponding to2.2" of water at the outlet side thereof.

Results of a typical run operating under the above conditions are givenin Table III.

Once again it will be observed that compared to the method of operationillustrated in Figure 1 the grade of concentrate is higher and thetailing losses lower. Com- 4 pared to the method illustrated in Figure 2the result is a slightly lower ratio of concentration, and slightlyhigher loss in the coarse tailings owing to the higher air velocityused.

Examples 2 and 3 illustrate the advantages which may be gained inincreased grade by the return of a coarse concentrate to the inlet sideof the milling circuit. Comparable advantage may be achieved in similarmanner on ores other than magnetic iron but which are subject to dryconcentration when in a state of sub-division larger than their primarygrain size. In the foregoing examples, magnetic concentration has beenutilized, but it will be appreciated that, where applicable, use may bemade of electrostatic concentration or any other form of dryconcentration which may suit the particular material being treated.

What I claim as my invention is:

1. A method of beneficiating magnetic iron ores containing aggregates ofsubstantially barren gangue particles and a non-uniform distribution ofaggregates of primary crystals of the mineral to be recovered, saidmethod comprising; passing the ore through a dry milling circuit,comprising a combined dry crushing and grinding mill and which isadapted to produce a substantial proportion of product in a size rangelarger than the primary crystal size of the contained mineral;collecting milling circuit product which is primarily of a particle sizelarger than the primary crystal size of said ore; subjecting saidcollected product to magnetic concentration to produce at least a coarseconcentrate; and returning the coarse concentrate to the inlet side ofsaid combined dry crushing and grinding mill.

2. A method of beneficiating magnetic iron ore of the type containingaggregates of substantially barren gangue particles and a non-uniformdistribution of aggregates of primary crystals of the mineral to berecovered whereby magnetic concentration is eifective to produce adiscardable tailing when the ore is in a state of sub-division largerthan its primary crystal size, said methodcomprising; passing the orethrough a dry milling circuit comprising a combined dry crushing andgrinding mill and which is adapted to produce a substantial proportionof product in a size range larger than the primary crystal size of thecontained mineral; collecting milling circuit product which is primarilyof a particle size larger than the primary crystal size of said ore;subjecting said collected product to magnetic concentration to produceat least a coarse concentrate and a discardable coarse tailing; andreturning the coarse concentrate to the inlet side of said combined drycrushing and grinding mill.

3. The method as defined in claim 1 wherein the magnetic concentrationstage produces a middliugs which is also returned to the inlet side ofthe milling circuit.

References Cited in the file of this patent UNITED STATES PATENTS1,637,648 Kopf Aug. 2, 1927 1,905,780 Ahlmann Apr. 25, 1933 1,931,921Breerwood Oct. 24, 1933 2,072,063 Alton Feb. 23, 1937 2,381,351 HardingeAug. 7, 1945 2,462,645 Knowland Feb. 22, 1949 2,595,117 Ahlmann Apr. 29,1952 2,627,375 Grondal et a1 Feb. 3, 1953 2,674,413 Weston Apr. 6, 19542,692,677 Bosqui et a1. Oct. 26, 1954 2,712,902 Kennedy July 12, 19552,835,452 Cline et al May 20, 1958 OTHER REFERENCES Handbook of MineralDressing, by Taggart, 1945, p. 2-141, published by John Wiley and Sons,Incorporated, New York,

1. A METHOD OF BENIFICIATING MAGNETIC IRON ORES CONTAINING AGGREGATES OFSUBSTANTIALLY BARREN GANGUE PARTICLES AND A NON-UNIFORM DISTRIBUTION OFAGGREGATES OF PRIMARY CRYSTALS OF THE MINERAL TO BE RECOVERED, SAIDMETHOD COMPRISING, PASSING THE ORE THROUGH A DRY MILLING CIRCUIT,COMPRISING A COMBINED DRY CRUSHING AND GRINDING MILL AND WHICH ISADAPTED TO PRODUCE A SUBSTANTIAL PROPORTION OF PRODUCT IN A SIZE RANGELARGER THAN THE PRIMARY CRYSTAL SIZE OF THE CONTAINED MINERAL,COLLECTING MILLING CIRCUIT